Hexagonal boron nitride (hereinafter called “boron nitride”), because of having lubrication capability, high thermal conductivity and insulation capability, is now widely used as releasing agents for solid lubricants, molten glasses and aluminum or the like as well as fillers for thermal radiation materials.
To be compatible with higher performances of recent computers and electronic hardware in particular, measures against thermal radiation have increased in importance and attention has been directed to the high thermal conductivity of boron nitride.
In recent years, it has been studied to add boron nitride to the resin layers of printed wiring boards and flexible copper-clad laminated sheets for the purpose of imparting high thermal conductivity and insulation to them.
While generally available boron nitride has an average particle diameter of a few μm to 20 μm, some resin substrates for printed wiring boards and flexible copper-clad laminated sheets have a thickness of the order of several tens μm, and large average particle diameters of boron nitride result in poor dispersibility in resins, failing to obtain smooth surfaces, or with that boron nitride dispersed, there are hard spots appearing, often making it impossible to keep the strength of the resin layer high. For these reasons, there is mounting demand for boron nitride fine particles of the submicron order (0.1 μm).
To have high thermal conductivity, the boron nitride must be of high purity and high crystallinity, and the same goes for boron nitrite fine particles on the submicron order (0.1 μm).
On the other hand, the boron nitride has a characteristic scaly shape, and its thermal properties are overwhelmingly better in the major or minor diameter direction than in the thickness or perpendicular direction. For this reason, the thermal properties of a composite material having boron nitride filled or packed in a resin such as silicone are considerably affected by the directionality of boron nitride fine particles in the composite material.
For instance when the composite material is provided in a sheet form, however, the boron nitride fine particles are often apt to lie down laterally and the necessary sufficient thermal properties are not obtained anymore in the longitudinal direction.
It follows that in order to be well fitted as a highly thermoconductive filler, the boron nitride must be configured into a spherical or aggregate shape thereby keeping the influence of directionality less.
The boron nitride is generally obtained by reactions at high temperatures between a boron source (boric acid, borax, etc.) and a nitrogen source (urea, melamine, ammonia, etc.), and a “pineal” boron nitride obtained by the aggregation of scaly primary particles from boric acid and melamine has been proposed in the art (Patent Publication 1).
However, the aggregate particle diameter of boron nitride prepared by this method is greater than 50 μm; in other words, it is difficult to prepare boron nitride fine particles of the submicron order—the object of the invention.
On the other hand, there have been reports (Patent Publications 2, 3 and 4) about how to obtain boron nitride fine particles by a vapor-phase synthesis process.
However, boron nitride fine particles obtained by these methods, because of having low crystallinity, are found to be less than satisfactory in terms of boron nitride's characteristics: lubrication capability and high thermal conductivity.